Elastic modulus engineering in β-titanium alloys: Tuning the precipitation kinetics using Sn

Design strategies for β-Ti alloys either aim at the suppression of the diffusion-assisted, isothermal ${\omega }_{iso}$ precipitation or at the controlled transformation of these precipitates into α phase by an ω-assisted route, with the kinetics of ${\omega }_{iso}$ formation playing an essential role in both cases. Therefore, controlling the ${\omega }_{iso}$ formation is pivotal in the design of β-Ti alloys with targeted properties. We propose that by controlling the Sn content added to β-Ti alloys a wide range of achievable microstructures for modulus engineering is accessible. Upon Sn addition, we observe an exponentially slowed down ${\omega }_{iso}$ formation based on blocking of diffusion pathways for the β-stabiliser atoms without the transformation being thermodynamically inhibited. This suppression of ${\omega }_{iso}$ formation allows the β-Ti alloys to maintain a lower elastic modulus necessary for biomedical applications. Furthermore, as byproduct, reducing the number density of ${\omega }_{iso}$ precipitates upon Sn addition also impedes the ω-assisted α-formation, while the Sn-free forms fine, acicular intragranular α plates and ${\alpha }_{GB}$ side-plates with similar morphology after low temperature pre-ageing. These results provide solid evidence for the previously proposed Sn-induced kinetic deceleration and suggest that the mechanical properties can be tailored as required by the application by tuning the ${\omega }_{iso}$ precipitation kinetics using Sn.